I cannot see anyone other than microsoft using DX12 exclusively on games, it makes no business sense, as they then cannot sell their games to the hundreds of millions of windows7/8 users out there.
The same reason why game developers still support windows XP today, they want to maximise sales.
Vulkan is different however as that is cross platform. So we may see games using vulkan only.
Also HL3 would never ever be DX12 only valve would use vulkan given they dont want people using the windows store.
W10 is not just a faster version of W7 as you put it.
W7 has no modern apps.
W7 doesnt force updates.
W7 does not change UI elements every few months on threshold builds.
W7 does not reset settings on updates undoing tweaks.
W7 is faster than W10 on my hardware.
W7 does not data mine file contents for telemetry, even tho telemetry has been added to W7 it is not in the same league as W10 telementry.
W10 telemtry is so bad it causes my dad's pc to slow down for what seems forever as its continuily thrashed his hdd and used cpu cycles. (noticeable on low end hardware).
W7 does not reboot without user interaction on updates.
W7 does not force new drivers onto the machine.
W7 does not have oversized Ui elements.
W7 does not integrate cloud functions (which I see as a benefit).
W7 does not require app developers to make a new version of app to keep it working on the OS when there is OS updates.
There is more differences then that as well, but I think I posted enough to make it clear what you posted was a tad naive.
With that said tho I dont use W7 anymore, some parts of it have become too dated, so I moved to W8.1 which I seen as a middle compromise, I get benefits of the new applocker security model, hyperV and able to use new modern apps if I want to, but without the downsides of the new updating system.
Microsoft's end game is still a locked down system, ultimately they want to kill win32 and have everything installed via their windows store. They want the apple model. Windows 10 will slowly transition to that state.
There's nothing wrong with preferring W7 or W8.1, as we all have a choice to install and use whichever OS we choose to. I do feel it's rather silly to exaggerate or flat out lie to try and argue your point, however.
I'll address some of your incorrect points:
1. There are already many games out there that don't support Windows XP. Examples include the incredibly popular Battlefield series, ever since the 2011 release of Battlefield 3. In fact the vast majority of AAA games don't work on XP anymore, so your point here is completely incorrect.
2. In the future, games will require DX12. This is inevitable. I'm not sure how old you are, though though of us who have been around the PC gaming scene for a few years already know that older DX versions are eventually made obsolete and are not used or supported. An example of which is Fallout 4, it is a Directx 11 only game, such are the vast majority of AAA game releases in 2016. I'd say that within the next year we'll start to see games require W10 to play. Each quarter that will go by from that point will increase the number of W10 only games. Then eventually, all AAA games will require W10, regardless of not if they are DX12 only.
3. Windows 10 is a faster operating system than W7 and W8.1. This has been proven in many reviews/benchmarks that have compared it to W7 and W8.1. It boots fater, handles resources faster and is simply a far more fluid experience than any Windows OS before it. Of course if your running on an old potato of PC, you'll see less difference, though you cannot deny that W10 is faster.
4. Modern Apps? I don't use any, they can even be easily uninstalled if you wish.
5. Why would you not want updates? Without them your system is left open to security threads. Nonetheless if, you truly wish to disable updates, this can also be done on W10, though I wouldn't recommend it.
6. Threshold builds. You claim they occur "every few months". Are you aware that there has only been one so far, in the 1 year of W10's history? Even so, it look me seconds to change a few settings back to my preferred options once I had upgraded to v1511
7. You system must be extremely old to not run faster on W10. I'd suggest checking if it meets the minimum requirements to install the OS, perhaps that's where your problems lie?
8. W10 can easily be configured to not collect any data. There is even a program called "Spybot Anti-Beacon" for those without the knowledge of how to disable the data collection themselves.
9. My PC takes around 6 seconds to boot with an NVME SSD. That said, I've never had it reboot without my intervention, though I'm in the habit of rebooting if I need to, as a 6 second boot time for me isn't a big deal. Perhaps if you are booting off a old 5400RPM HDD this could be a nuisance, though in this day and age, SSD's are affordable and ubiquitous.
10. W10 can easily be configured to not install new drivers, it's a less than 10 second process.
11. I'm pleased with how the UI elements of W10 scale on my monitors. My main monitor is a 1440P screen, though my 1080P laptop also feels and looks great. Very similar to W7. These settings can also be customised if you so wish.
12. Cloud functions? I assume you mean onedrive? This can also be disabled very easily if you don't like it, though obviously data is only stored in the one drive cloud if you would transfer data there, an action which requires you to do so yourself.
13. Updating Apps is a fairly common and needed scenario, as windows updates improve security and have performance features.
Anyway, all this is besides the point. Everyone will have to upgrade to W10 eventually, as Microsoft's support for older OS's is counting down by the minute:
Windows XP: Mainstream and extended support has ended.
Windows Vista: Mainstream support has ended. Extended support ending in 2017
Windows 7: Mainstream support has ended. Extended support ending in 2020
Windows 8/8.1 Mainstream supports ends in 2018. Extended support ending in 2023
Time is ticking, though I imagine the vast majority of current PC users will be tablet users by 2023, making our discussion rather pointless